An optical head focuses a light beam as a small spot on a recording track formed on a information recording surface of an optical disc, serving as a recording medium, and records, reproduces and erases in accordance with the variation of a light reflected from the information recording surface. Therefore, the light beam should be accurately focused on the recording track at all times, in spite of disc warp and disc vibration caused by the rotational movement of the disc. Focusing of the light beam is controlled by plant displacement of the objective lens in the direction of its optical axis (focus servo, hereinafter).
Since the light spot should precisely trace the recording track at all times, the position of the objective lens is minutely controlled also in a direction, being perpendicular to the recording track (tracking servo, hereinafter).
Technologies on the focus and tracking servos are disclosed on Japanese Patent Kokai 5-68013, 4-319537, 62-65243 and 4-324127.
An apparatus playing the most important role in the above mentioned control of the light spot is an objective lens actuator. The important structural elements of this apparatus are an objective lens holder and four supporting members for said lens holder. The objective lens holder consists of an objective lens mounted thereon and a coil assembly, which generates electromagnetic forces for servo drive. One end another ends of a supporting member are fixed to the objective lens holder and an actuator base, respectively. In many cases, the supporting member is an elastic wire or an elastic leaf spring. A very important problem is hunting of the objective lens holder, which could arise in servo-drives of the objective lens holder. Then, improvements in damping characteristic of the movements of the objective lens holder are extremely important, and considerable efforts should be made to improve not only the construction of the supporting system, but also the structure of the supporting member itself from this point of view.
Another important problem is the remaining mechanical stress in each supporting member due to the heat in a soldering process. The attitude of the optical lens to the actuator base becomes in accurate of account of said remaining mechanical stress. Then, the improvement of the soldering process is very important.